Manufacturing of spiral bevel and hypoid gears may be conducted according to the methods which include the following:                A. Continuous indexing face hobbing with a circular face cutter, which rotates while the work also rotates such that consecutive blade groups move through consecutive slots. The cutter rotates, in addition to the rotation around its center, around the axis of a theoretical generating gear (i.e. the generating roll). The correct tooth is formed if the work also rotates with the correct ratio in order to stay in mesh with the theoretical generating gear (generated bevel gear).        B. Continuous indexing face hobbing with a circular face cutter, which rotates while the work also rotates without any generating roll motion. The cutter represents one tooth of the negative image of the work (non-generated bevel gear). This process is usually applied to ring gears which are mated with pinions that have been manufactured using a generating gear which is identical to the ring gear.        C. Single indexing face milling, with a circular face cutter, which rotates while the work is not performing any indexing motion. The cutter represents one tooth of the generating gear which, in addition to its rotation around its axis, rotates around the theoretical generating gear axis (i.e. the generating roll). The correct tooth is formed if the work also rotates with the correct ratio in order to stay in mesh with the virtual generating gear (generated bevel gear).        D. Single indexing face milling, with a circular face cutter, which rotates while the work is not performing any indexing motion. The cutter represents one tooth of the negative image of the work (non-generated bevel gear). This process is usually applied to ring gears which are mated with pinions that have been manufactured using a generating gear which is identical to the ring gear.        
Methods B and D are fast because the time for the generating motion is saved. As a result, the tooth form has no involute shape in profile and also is not wound around the pitch cone. Although the tooth form of the mating member accounts for the special, simplistic shape of the non-generated bevel gear, flank form corrections in tooth length direction have been very difficult or even impossible.
One flank form correction type which is necessary in order to allow a very conjugate flank center (achieved by little or no crowning in tooth profile direction and the tooth length direction) is the border relief (or end relief).
In the profile direction it is common to apply a protuberance to the cutting blades of both mating members which will provide an ease-off strip in the transition zones between the tooth flanks and root fillets. This correction type can be applied to methods A, B, C and D.
End relief in the face width direction is preferred to begin parallel to the toe (inside) or heel (outside) tooth border (or perpendicular to the pitch line or root line). Generating processes can only influence the tooth form (and therefore any kind of relief) beginning at a line which is parallel to the generating marks but which is oriented at an angle with respect to the pitch line of a tooth. This angle varies with the spiral angle and is in most common cases below 45 degrees. Therefore, the desired end relief cannot be achieved with generating processes.
In the tooth length direction (face width), a special cutter head design in connection with a special machine movement is known for Method D. The different blade groups in the cutter head had to be assembled with different blade stick outs and when the last blade for the convex flank entered the slot at the toe end, the cutter spindle had to perform a fast axial move in order to remove, in a region of 3 to 6 mm from the toe border in tooth length direction, more material than in the remaining slot. Slot roughing had to be done in a separate step with a conventional cutter head on a conventional machine. The finishing was done on a machine with a very slow rotating cutter spindle (with the described axial motion) in one cutter revolution per slot (broaching process). The spaces between the last finishing blades and the preceding blades had to be larger than the face width of the gear in order to avoid cutting action of other blades during the corrective axial motion occurred.
In today's high speed cutting of bevel gears, method D is no longer used. Surface speeds of coated carbide cutters are more than 10-times faster than the “broaching” process which would require axial cutter movements between 5 and 15 Hz, versus 0.5 Hz in the broaching process. Such a high frequency is difficult to realize and would cause significant dynamic process disturbances which reduce the tool life and lead to poor part quality. Also, a cutter head with blade spacing larger than the parts face width would not be very productive.
When the broaching process was applied for bevel gear cutting, the hard finishing operation for face milled bevel gears was commonly lapping. The relief allowed lapping the flank centers to be very conjugate while the end relief from the soft cutting operation was still present after the lapping and prevented toe edge contact under high load and deflection in operation. Today, about 75% of all face milled bevel gears are hard finished by grinding. Since a grinding wheel has a rotationally symmetric abrasive surface with continuum of cutting edges (i.e. abrasive grains) which are spaced about at a fractional millimeter distance, the creation of an end relief as described above is physically not possible.